Polar solvation and solvation dynamics in supercritical CHF ₃: Results from experiment and simulation

Solvation dynamics of the probe trans-4-(dimethylamino)-4'-cyanostilbene (DCS) have been measured in supercritical fluoroform at 310 K (1.04 T _c) and solvent densities over the range 1.4-2.0 _pc using optical Kerrgated emission spectroscopy. Steady-state measurements and computer simulations of this and the related system coumarin 153 (C153) in fluoroform are used to help interpret the observed dynamics. The solvent contribution to the Stokes shift of DCS is estimated to be 2300 ±400 cm ^-1 and nearly density independent over the range (0.7-2.0) _pc. Spectral response functions are bimodal and can be fit to biexponential functions having time constants of ∼0.5 ps (85%) and 3-10 ps (15%) over the observable range ((1.4-2.0) _pc). Computer simulations based on a 2-site model of fluoroform' and assuming an electrostatic solvation mechanism appear to properly account for the magnitude and weak density dependence of the Stokes shifts but predict much faster solvation than is observed. Possible reasons for the discrepancy are discussed.